Spectrally broadband instruments like densitometers and most colorimeters serve a variety of useful applications, though their usefulness is limited to a dedicated output or small sets of outputs. When only a measurement of RGB density is required, a densitometer will provide a direct and expedient result without the need for any form of intermediate data. Likewise, if the CIELab values of a sample or group of samples is all that is needed, or perhaps the XYZ tristimulus values of those samples, an inexpensive colorimeter can provide those results. These devices make use of the net spectral content of the colors being measured and inherently reduce the information to only a few values, usually three. When those three values are all that is needed, these devices have served their purpose.
By comparison, spectrometric devices (e.g. spectrophotometers and spectroradiometers) deliver data at the opposite extreme. Instead of providing a specific final result in only one metric, they provide the most fundamental measurement of a sample's color—it's spectral content. The spectral information, once collected and recorded, can be used to calculate virtually all other metrics used to quantify colors, including analytical density and colorimetric units such as XYZ, CIELuv, and others. It should be noted, however, that other data may be required along with the spectral measurements such as data pertaining to the colorants in order to perform the necessary calculations. Beyond the versatility advantages to collecting spectral data, there is the added advantage of having the ability to characterize spectral phenomena such as metamerism and fluorescence.
Conventional spectrophotometers employ a light source, a detector, and a device that is used to disperse or diffract light reflected from or transmitted through a sample, thereby allowing the spectral components to be detected and quantified. Commonly used components include prisms and diffraction gratings. Instruments that disperse the light source into it's spectral components, and then measure the amount of that light that is reflected from or transmitted through a sample, are known as monochromators.
Prisms are usually made from transparent materials such as glass or plastic, and disperse light into its spectral components. The material from which the prism is made has a refractive index that is different from that of air. When light passes from one material into another, it is refracted, that is to say its direction is changed by an amount that is dependent upon the difference between the refractive indices of the two materials. Further, the angle of refraction varies with wavelength so the spectral components that comprise the light are dispersed in space. A single sensor can be translated across this spatial dispersion to measure the amounts of each spectral component, or a stationary linear sensor can be positioned to measure the spectral components all at once.
Like prisms, diffraction gratings also disperse light into its spectral components, although the mechanism is very different. A diffraction grating is a reflecting or transmitting element that consists of a series of fine, parallel, equally spaced slits or rulings (grooves) on a material surface. When light passes through such an element, a pattern is produced by Fraunhofer diffraction. The advantage of gratings over prisms is the high resolving power they afford.
When light consisting of a single wavelength, i.e. monochromatic light, is passed through a diffraction grating, analysis of the resulting diffraction pattern can be used along with knowledge of the spacing between the rulings to determine the wavelength of the light. If the light consisted of two different wavelengths, then two patterns would be formed and the two separate wavelengths could subsequently be determined. If white light was passed through the diffraction grating, each wavelength would be sent in a different direction, as defined by the grating equation, and the pattern would appear as a spectrum. The amount of energy at each wavelength could be determined at a spectral resolution limited by the spacing of the rulings. This spacing is referred to as the grating space (d). The narrower this spacing is, the more widely the spectrum is spread.
High quality diffraction gratings are fabricated by ruling fine grooves with a diamond point either on a plane glass surface to produce a transmission grating, or on a polished metal mirror to produce a reflection grating. The grooves scatter light and are effectively opaque while the undisturbed parts of the surface transmit or reflect light regularly and act as slits. The most fundamental requirement for a good diffraction grating is that the lines must be as equally spaced as possible across the entire surface of the grating, which may be up to 25 cm in width. After each groove has been ruled, the diamond point must be lifted and moved to the location of the next groove, and few ruling machine exist to meet this difficult requirement. Consequently, high quality rule diffraction gratings can be quite expensive. Photolithographic techniques have been developed which allow gratings to be created from holographic interference patterns. Holographic gratings have sinusoidal grooves and are, therefore, not as efficient as ruled gratings, though they have much lower fabrication costs.
A third type of device that can be used to disperse light into its spectral components, is the Fabry-Perot interferometer or etalon. To be precise, the former term refers to a device that uses two parallel highly reflecting mirrors while the latter is a transparent plate onto which has been deposited two reflecting surfaces, though the two terms are often used synonymously. The device is named after Charles Fabry and Alfred Perot. Etalon is from the French etalon, meaning “measuring gauge” or “standard”.
Generally speaking, interferometry is the science of superimposing or interfering two or more input waves to create a different output wave that can subsequently be used to obtain information about the differences between the input waves. It is based on the principle that two waves that coincide with the same phase will add to each other while two waves that have opposite phases will cancel each other out when both waves have the same amplitude. The varying transmission function of an etalon is caused by interference between the multiple reflections of light between the two reflecting surfaces. Constructive interference occurs when the transmitted waves are in phase, resulting in high transmission. Destructive interference occurs when the transmitted waves are out of phase, resulting in low transmission. Whether the multiply-reflected waves are in phase depends on the wavelength of the light (λ), the angle the light travels through the etalon (θ), the thickness of the etalon (l), and the refractive index of the material between the reflecting surfaces (n).
The relationship between the wavelength of the light and the angle that it travels inside the etalon, for each maxima (m), is given by:2nl cos θ=mλ